The invention relates to a process for the polymerisation of one or more monomers in a fluidised bed reactor, which reactor comprises a reaction zone which is confined at the underside by a gas distribution plate and at the top side by a virtual end surface, in which a fluidised bed is maintained between the underside and the top side and in which at least part of the gaseous stream withdrawn from the top of the reactor is cooled to a point where the stream partially condenses into a liquid, and in which at least part of the resulting two-phase stream is recycled to the reactor via an inlet which terminates in the reactor below the gas distribution plate.
Gas-phase fluidised bed polymerisation of one or more monomers, like an olefin or olefins, is effected in a usually vertical elongated reactor in which a bed of polymer particles is maintained in fluidised condition with the aid of an ascending gas stream which contains at least the gaseous monomer(s) to be polymerised. The gas stream is passed through a gas distribution plate which separates the lower part of the reactor from the reaction zone proper. In this plate there are provided perforations that suitably distribute the gas stream supplied over the reaction zone. A peripheral section of the gas distribution plate may be sealed so as to obtain a particular pressure drop at a lower flow rate of the gas. In order to prevent polymer particles from building up on such peripheral section, the seal is preferably designed as an inclined wall which extends from the gas distribution plate to the wall of the reactor. The angle of the inclined wall to the gas distribution plate must be greater than the angle of natural repose of the polymer particles in the reactor and, furthermore, is generally at least 30xc2x0, preferably at least 40xc2x0 and more preferably is between 45xc2x0 and 85xc2x0.
The ascending gas stream may optionally contain one or more inert gases and for example hydrogen as a chain length regulator. An important objective of the addition of inert gases is to control the dew point of the gas mixture. Suitable inert gases are for example inert hydrocarbons such as (iso)butane, (iso)pentane and (iso)hexane, but also nitrogen. Such an inert gas may be added to the gas stream as a gas or, in condensed form, as a liquid.
The gas stream is discharged through the top of the reactor and, after certain processing operations, fresh monomer is added to it to make up for the monomer(s) consumed in the polymerisation, and then the gasstream is again supplied to the reactor as (a portion of) the ascending gas stream in order to maintain the bed.
A catalyst is also added to the bed. During the process, under the influence of the catalyst present, fresh polymer is continuously formed and at the same time polymer that has formed is withdrawn from the bed, with the bed volume and mass being kept substantially constant.
The polymerisation is an exothermic reaction. Heat needs to be removed continuously so as to keep the temperature in the reactor at the desired level. Such removal is effected via the gas stream which leaves the reactor at a higher temperature than that at which it is supplied to the reactor. The superficial gas velocity in the reactor cannot be chosen to be arbitrarily large and so no arbitrarily large amount of heat can be removed. The minimum velocity is dictated by the requirement for the bed to remain fluidised. On the other hand, the velocity must not be so large that a significant amount of polymer particles are blown out through the top of the reactor. The aforementioned limits are heavily dependent on the dimensions and the density of the polymer particles present in the bed and can be determined by experiment. Practical values for the superficial gas velocity are between 0.05 and 1.0 m/sec. These requirements are elements which limit the maximum flow rate of the gas stream at the given reactor dimensions and, thus, the maximum attainable heat removal. The maximum allowable amount of heat of reaction produced, and hence the maximum amount of polymer to be produced, are limited likewise.
The detailed design and operation of fluidised bed reactors for the polymerisation of one or more olefin monomers and suitable process conditions are known per se and are described in detail in for example U.S. Pat. No. 4,543,399 and in WO-A-94/28032.
From that same U.S. Pat. No. 4,543,399 it is known to replenish the gas stream discharged from the reactor with fresh monomer(s) and to cool it to a point where the stream partly condenses (the so-called xe2x80x9ccondensed modexe2x80x9d). The two-phase stream so obtained, which because of the latent heat of evaporation of the liquid phase has a substantially larger heat removal capacity, and so a corresponding cooling capacity, than a stream consisting solely of a gas, is recycled to the bottom of the reactor. The dew point of the two-phase stream must be lower than the temperature in the reaction zone so that the liquid can evaporate in it. In this way, the production capacity of a fluidised bed reactor appears to be substantially higher than that of reactors which use a recycle gas without condensed liquid, said reactor having otherwise equal dimensions. In the known process the maximum amount of liquid in the two-phase stream is 20 wt %. The highest figure quoted in the examples is 11.5 wt %.
From WO-A-94/28032 it is known to separate the liquid from the two-phase stream obtained on cooling of the gas stream to be recycled and to feed said liquid to the reactor separately from the gas stream. The liquid is preferably injected or atomised at a certain height into the fluidised bed proper, optionally with the aid of a gaseous propellant. In this way, according to this publication, it is possible to feed a larger amount of liquid in proportion to the amount of gas being fed. This allows an even larger amount of heat to be removed, so allowing higher polymer production with proportionally higher heat production. WO-A-94/28032 quotes a figure of 1.21 as the maximum permissible ratio of the mass of liquid feed to the mass of the total gas feed, which figure was derived from a simulated experiment.
The present invention relates to a process for the polymerisation of one or more monomers in a specific fluidised bed reactor, which reactor, at given dimensions, allows a higher liquid mass to gas mass ratio in the feed to the reactor than in a reactor according to the prior art, both in cases where the reactors are operated under xe2x80x9ccondensed mode conditionsxe2x80x9d.
This object is achieved by a process in which the reaction zone of the reactor is divided into two or more compartments by one or more substantially vertical partition walls extending from a point located above the gas distribution plate to a point located below the end surface.
It has been found that when in such a reactor a fluidised bed is maintained that extends, both at the top and bottom, beyond the partition walls, so that the partition walls are submerged in the fluidised bed, more liquid can be supplied in proportion to the total gas feed than in the absence of a partition wall. This increases the heat removal capacity of the process, so allowing higher heat production and hence higher polymer production rates at equal reactor dimensions. Even at a constant liquid to gas mass ratio in the feed to the reactor, the process of the present invention results in a higher productivity of the reactor.
In a reactor according to the prior art the ratio of the height (H) of the fluidized bed to the diameter (D) of the radial cross section (H/D-ratio) usually is 3 to 5 at the most. At higher ratios it has proved impossible to maintain a stable fluidized bed if, besides gas, liquid is fed to the reactor.
An additional advantage of a reactor having at least one partition wall is that it is now possible to choose a higher H/D-ratio for the reactor, for instance, an H/D-ratio of greater than 5, and even up to 20, which is much higher than in the case of the known reactors, while yet maintaining a stable fluidised bed, resulting in a more controlled polymerisation process. This advantage presents major engineering advantages for polymerisation reactors because they are pressure vessels.
A particularly suitable partition wall in the reactor of the invention is a pipe or hollow section placed in vertical position, preferably concentric with the reactor. Since the pipe or hollow section is completely submerged in the fluidised bed, no appreciable pressure differences occur across the wall of the pipe so that the pipe may be of light-duty construction. This applies also to walls of different shapes.
The walls can simply be suspended from a higher section of the reactor, supported by a bottom section or secured to the wall of the reactor. In the present context a hollow section differs from a pipe in terms of the shape of its cross section. The cross section of a pipe is curved, for example circular or elliptical, whilst that of a hollow section is angular, for example triangular, rectangular, octagonal or with more angles, with or without the angles being uniformly divided. The hollow section or the pipe may have a uniform and/or tapered cross-section, for instance a cone shape, including tapering inwardly and outwardly, for instance, in a hyperbolic shape. For conical shapes, it is preferred that the apex angle formed by the walls of pipe or hollow section is generally not more than 5xc2x0, preferably not more than 2.5xc2x0. Particularly suitable are angles between 0xc2x0 and 2xc2x0. The ratio of the area of the radial cross section of the pipe or hollow section to that of the reactor is between 1:9 and 9:10 and, in order to achieve as high a stability as possible, preferably between 1:5 and 3:4. In the case of a conical pipe or hollow section, the same applies to the average cross-sectional area thereof. The lower end of the pipe or hollow section is located at least 0.1xc3x97the diameter of the reaction zone above the gas distribution plate and preferably 3xc3x97that diameter at the most. If the dimensions given here are departed from, the favourable effect of the presence of a vertical partition wall is diminished. The upper end is located at least 0.1xc3x97the diameter of the reaction zone below the end of that reaction zone and preferably not more than 3xc3x97that diameter. It has been found that it is far less critical for the bed to extend further beyond the partition wall at the upper end than at the lower end. The upper end of the partition wall may be lower accordingly as the H/D-ratio of the fluid bed increases. What is stated here on the positioning of the wall in the reaction zone applies also to the vertical partition walls to be explained below.
Another embodiment of a suitable partition wall is a substantially axially oriented flat, curved or folded plate present in the reaction zone. It is preferred for such a partition wall to connect to the inner wall of the reactor although a clearance of up to 10 cm in-between is permissible. In this way, the reaction zone is divided into two or more compartments, which may be differently sized. The area ratio of the radial cross section of a compartment to the radial cross section of the reactor preferably is between 0.1 and 0.9 and more preferably between 0.20 and 0.75. The substantially axially oriented wall should be virtually vertical. Preferably, the partition wall is oriented substantially parallel to the longitudinal axis of the reactor. This should be understood to mean parallel with the axis of the reactor in its vertical position but also out of parallel by not more than 5xc2x0, preferably not more than 2.5xc2x0.
The aforementioned beneficial effects of a partition wall occur when there is a common inlet for a gas/liquid mixture at the underside of the reactor, as described in U.S. Pat. No. 4,543,399, and also when there is a separate gas and liquid inlet in the fluidised bed, as described in WO-A-94/28032.
In the latter case, the liquid may be supplied to the fluidised bed via the underside of the reactor at one or more points through the gas distribution plate as well as at one or more points through the side wall. It is in any case advantageous to arrange the means of introduction of the liquid in such a way that the bulk of the liquid can be supplied into the fluidised bed in a zone located under or in the central compartment if a pipe or hollow section is employed, or under or into one of the compartments if one or more vertical partition walls are present. In the case of introduction of the liquid via one or more points through the side wall of the reactor, and if the partition wall is a pipe or hollow section, it is advantageous to position the means of introduction so that the liquid can be supplied to the fluidised bed at a point below the lower end of the pipe or hollow section. In that case, for example by suitably choosing the feed velocity, the liquid can be supplied to both the central compartment and the peripheral compartment of the reactor. Preferably, the bulk of the liquid is supplied to the central compartment, located within the pipe or hollow section, inasmuch as the best results are obtained herewith.
In the case that a vertical plate is used as a partition wall, the liquid may be introduced in the aforementioned manner from a height below the lower end of the wall but also via inlets arranged at different heights in the section of the reactor wall which confines the compartment or compartments to which the liquid is to be supplied.
The liquid is preferably injected in finely divided form, preferably in atomised form, optionally with the aid of a propellant, for which purpose for example recycle gas or fresh monomer gas may be used. Injection should take place in such a way that the liquid enters the desired compartment whence it is taken up by an ascending fluidising gas stream. This has been found to be favourable in terms of the amount of liquid that can be supplied to the fluidised bed without sintering of polymer particles or other undesired disturbances occurring in the bed.
Introducing the recycle liquid via several inlets at different heights of the reactor gives the possibility to vary the concentration of the different ingredients of the liquid inlet (through the addition of more or less monomer make up, etc.) which improves the operating window of the polymerisation reaction and therefore broadening the product capabilities of the fluidised bed reactor.
In processes in which the H/D-ratio of the reactor exceeds 5, the means of introducing the liquid may also be positioned above the upper end of the partition wall(s) provided that the distance between the said upper end and the top of the fluidised bed is not less than approx. 2 m.
The process of the present invention has been found to allow the benefits related to the new reactor design to fully manifest themselves. In the process of the invention the reactor can be operated in a stable manner even when the mass ratio of (liquid supplied to the reactor):(amount of gas supplied to the reactor) is higher than 2:1 or even higher than 4:1. The aforementioned ratio is in any case at least 10% and even more than 50% to even more than 100% higher than when the process is operated in a similar reactor without partition wall(s).
The supplied amount of gas includes, besides the gas supplied via the recycle stream, all other gases supplied to the reactor, including at least the propellant and carrier gases that are employed in introducing the catalyst, a catalyst activator and/or other substances desired or needed for the polymerisation and those used for atomising the supplied liquid.
The process according to the present invention is suitable for any kind of exothermic polymerization reaction in the gas phase. Suitable monomer include olefin monomers, polar vinyl monomers, diene monomers and acetylene monomers. The process of the present invention is especially suitable for the manufacture of polyolefins by the polymerisation of one or more olefin monomers, at least one of which is preferably ethylene or propylene. Preferred olefin monomers for use in the process of the present invention are those having from 2 to 8 carbon atoms. However, small quantities of olefin monomers having more than 8 carbon atoms, for example 9 to 18 carbon atoms, can be employed if desired. Thus, in a preferred mode, it is possible to produce homopolymers of ethylene and/or propylene or copolymers of ethylene or propylene with one or more C2-C8 alpha-olefin monomers. The preferred alpha-olefin monomers are ethylene, propylene, butene-1, pentene-1, hexene-1, 4-methylpentene-1, and octene-1. An example of a higher olefin monomer that can be copolymerised with the primary ethylene and/or propylene monomer, or as partial replacement for the C2-C8 monomer is decene-1. Also dienes are suitable, like 1,4-butadiene, 1,6-hexadiene, dicyclopentadiene, ethylidene norbornene and vinyl norbornene.
When the process is used for the copolymerisation of ethylene and/or propylene with other alpha-olefin monomers the ethylene and/or propylene are present as the major component of the copolymer, and preferably are present in an amount at least 70 wt %, more preferably 80 wt % of the total monomers.
The process is particularly suitable for polymerising olefin monomers at a pressure of between 0.5 and 10 Mpa, preferably between 1 and 5 Mpa, and and at a temperature of between 30xc2x0 C. and 130xc2x0 C., and particularly between 45xc2x0 C. and 110xc2x0 C.
The polymerisation reaction may be carried out in the presence of any catalyst system known in the art (for instance, anionic catalyst, cationic catalyst or free-radical catalyst) suited for the gas phase polymerisation of one or more (olefin) monomers, like a catalyst system of the Ziegler-Natta type, consisting of a solid catalyst essentially comprising a compound of a transition metal and of a cocatalyst comprising an organic compound of a metal (i.e. an organometallic compound, for example an alkylaluminium compound); also so-called single site catalyst systems, like metallocene based catalyst systems, are suitable.
The catalyst may also be in the form of a prepolymer powder prepared in a prepolymerisation stage with the aid of a catalyst system described above. The prepolymerisation may be carried out by any known process, for example, polymerisation in a liquid hydrocarbon diluent or in the gas phase using a batch process, a semi-continous process or a continous process.
The invention also relates to a reactor system, suitable for carrying out the process of the present invention. Such a reaction system comprises a fluidised bed reactor, having at the underside a gas distribution plate, having means for the supply of reaction ingredients, having means for withdrawal of a gaseous stream from the top of the reactor, having a cooler/condensor for cooling said gaseous stream to a point where the stream partially condenses into a liquid, and having means for recirculating the stream out of the cooler/condensor to the reactor.
Such a reactor system is known from the art cited above.
The aim of the invention is to provide a reactor system, in which a process for the polymerisation of one or more (olefin) monomers is possible, in which system a higher condensed mode can be applied.
This is achieved in a reactor system, wherein in the reactor the reaction zone is divided into two or more compartiments by one or more substantially vertical partition walls, extending from a point located above the gas distribution plate to a point located below the virtual end surface of the fluidised bed under polymerisation conditions.
In particular, said partition wall is a pipe or hollow section, preferably concentric with the reactor. The preferred configurations of the reactor system of the invention are described in greater detail earlier in this specification. In particular, the reactor system of the present invention comprises means for recirculating the stream out of the cooler/condensor to the reactor as a gas/liquid mixture. In another preferred mode, the reactor system also comprises a gas-liquid separator to separate at least part of the condensed liquid out of the resulting two-phase stream from the cooler/condensor and means for introducing at least part of the separated liquid into the fluidised bed reactor.
It should also be appreciated that the present invention is suitable for retrofitting existing reactors by installing one or more partition walls, pipes or hollow sections into the reactor. In particular, a reactor could be retrofitted by installing a pipe, as discussed above, by fixedly attaching the pipe to an internal section of the reactor.
It should be appreciated that retrofitting refers to the process of modifying or otherwise altering a previously utilized reactor, preferably a reactor previously utilized for polymerization reactions and more preferably condensed mode polyolefin polymerization reactions.
The invention is applicable both for so-called xe2x80x9cgrass-rootxe2x80x9d installations as well as for debottlenecking existing fluidised bed polymerisation installations. In the last case the full benefit of the invention might not be obtainable, as the capabilities of other units in the total polymerisation system may form a constraint on the maximum productivity of the system. (In other words: the throughput of the polymerisation system as a whole might be hindered by constraints in the system other than in the reactor section.) In situations where a new, integrated, polymerisation process is designed and built (xe2x80x9cgrass rootxe2x80x9d), the benefits of the present invention can be fully used and exploited.